It is now common in the automotive and other industries to fasten large parts to one another with structural adhesives which act as both weld and sealant, for example to join body panels. This is often done on an assembly line via automated, robot-mounted dispenser nozzles moving at fairly high speed over the workpiece.
Adhesives applied in this manner are typically of high viscosity and are often semi-rigid at room temperature, and must be applied at both high temperature and high pressure to flow properly from the nozzle onto the workpiece.
There are two primary methods for applying high viscosity adhesives, sealants, epoxies and the like (hereinafter "adhesives") from a moving dispenser nozzle: "extrusion" and "streaming". "Extrusion" is illustrated in FIGS. 1 and 2, and occurs where tool speed exceeds the velocity of adhesive flow from the nozzle at a relatively low order of magnitude, for example 2:1. The nozzle must be kept close to the work surface, for example on the order of 3 mm (roughly 1/8") where it forms an initial bead size which settles or flattens out in the wake of the dispenser nozzle to a desired bead size, for example 2 mm. The nozzle diameter typically approximates the desired bead size.
"Streaming" is a higher speed application with the nozzle spaced much farther from the workpiece, for example on the order of 50 mm (2 inches) rather than 3 mm (1/8 inches). In order to stream the high viscosity adhesive, it is necessary to increase its temperature, increase pressure, and reduce nozzle diameter to as low as 1/16 or 1/20 the diameter of the desired bead size so that it flows or "streams" freely like a liquid.
A factor to be considered in the automated application of such adhesives is whether the adhesive is a one-part or two-part material. One-part adhesives are simpler to apply, and lend themselves to high speed streaming applications. Two-part adhesives are known to form a stronger bond, but require mixing of the component parts just prior to being applied to the work surface. Mixing is typically accomplished through a relatively long mixing tube which receives the pressurized, temperature-conditioned components of the adhesive separately at its upper end and, via a helical arrangement of internal mixing plates, mixes the components by the time they reach the nozzle or outlet end of the tube. Mixing requires a relatively long tube which takes up valuable space at the end of the tool or robot arm; requires more complicated pressure regulating equipment, valving, and preheating equipment, which is too bulky to fit on the end of the robot arm with the mixing tube and must be located remotely; and tends to cause the two-part mixture to set inside the tube before it is dispensed. Since most automated assembly line dispensing applications require high speed and close control over adhesive pressure, temperature and volume, the poor quality of tube mixing has resulted in two part adhesives being largely ignored for streaming-type applications and confined to lower velocity "extrusion" applications.
State of the art mixing tubes are long, slender (usually plastic) tubes which are relatively inexpensive to replace when clogged, and which are useful for precise application of adhesive in confined areas. However, these same advantages make them relatively weak and prone to bending or damage, which at a minimum throws off the precision of the applied adhesive pattern.
A related problem with automated adhesive dispensing, especially at high speed, is the need to maintain constant bead uniformity and volume over the workpiece. For example, a perimeter bead of adhesive applied to an automobile door panel needs to be applied in just a few seconds on a high speed assembly line. As the adhesive bead is applied around the perimeter of the door panel, it undergoes several direction and velocity changes, decelerating into turns and accelerating out of turns. Again, the critical nature of volume control under these conditions has limited use to streaming-friendly one part adhesives.